Subtle Molecular Changes Induce Rare Chiral Phases
Another dive into the fundamental studies that researchers use Fluorochem’s chemicals to undertake: an in-depth look at the link between chemical structural features and macroscopic properties in liquid crystal phases. Ewan Cruickshank et al investigated the effect of single atom changes on the elusive twist-bend nematic (NTB) and smectic C (SmCTB) phases.
Exhibiting local chirality, despite being composed of achiral molecules, the twist-bend nematic phase is a topic of great interest. Within nematic phases, molecules are free to move, but remain aligned in a preferred orientation; however, within the NTB phase, that preferred orientation (or ‘director’) rotates through the phase, creating a heliconical structure. Establishing what molecular features drive this spontaneous generation of chirality is a high-priority in the field – with a bent molecular shape established as an essential requirement. In Smectic C phases, molecules are arranged within layers, tilted at an angle to the stack. Analogously, a twist-bend form, where this tilt changes between layers, exhibits chirality despite its achiral components. Again, these phases are stabilised by bent molecules – usually asymmetric dimers of two different mesogens (compounds that form liquid crystal phases) linked by a flexible spacer.
Schematic illustration (left to right) of nematic phase, twist-bend nematic phase, layered smectic C phase and twist-bend smectic C phase
By creating two new series of liquid crystals, CBO5O.Om, CBO5O.Sm and expanding the CBO5O.m and CB6O.Om series as well as comparing their properties along with those reported for the CB6O.m and CB6O.Sm series Cruickshank et al were able to build structure property relationships and better understand how these small changes affect phase behaviour. Interestingly, all series could form the optically biaxial twist-bend smectic C Phase SmCTB-SH at low temperatures. Across all four of the new and expanded series, the importance of chain length and terminal chemistry in tuning the balance between phases was apparent: longer terminal chains (and therefore more linear molecules) and more linear linkages led to a suppression of NTB phases, relatively stabilising Smectic A (SmA) and SmCTB-SH phases.
Studying the nematic-isotropic clearing temperature (TNI) across the series shows increasing chain length (and the consequent dilution of mesogenic interactions) leads to loss of nematic phase stability, while still exhibiting an odd-even effect, with odd-numbered chains showing greater variability. The linker chemistry was shown to be highly important, with the more linear oxygen linked dimers having the highest TNI values, followed by methylene links and finally the more angled sulfur links showing a ~40 °C drop in TNI relative to their O-linked analogues. However, a clear exception to this is found with the very shortest terminal chains, of only two atoms length. Here, the sulfur-linked series show a higher value for TNI than either of their methylene linked analogues, despite a smaller bond angle. In this case, a further interaction is presumably stabilising the mesogenic properties. Possibly chalcogen bonding is responsible, but crystal structures of the CB6O.Sm do not reveal any S-S interactions in the solid phase. Similar properties have been observed in other mesogens with terminal alkylthio groups, the affect being attributed to greater dispersion forces from the polarisable sulfur atom. Whatever the cause of the effect, such a large change in transition temperatures with a single atom swap in these short chain compounds is remarkable, but mysterious.
Top left: rope-like texture of 40:60 mol% mixture of CB7CB:CBO5O.O3 in NTB phase (T =80°C); top right: parabolic texture of 40:60 mol% mixture of CB7CB:CBO5O.S3 in NTB phase (T =81°C); bottom left: pseudo-schlieren texture with moving stripes of opposing birefringence of CB6O.O15 in STB-SH phase (T =90°C); bottom right: focal conic fan texture, in planar aligned cell, of CBO5O.S15 in STB-SH phase (T =85°C) (reproduced with authors’ permission)
Similar trends are observed in the NTB – N transition temperatures: sulfur lowers TNTBN by ~27 °C relative to oxygen, but only ~7 °C relative to carbon. With respect to the SmCTB-SH phase, onset temperatures increase with chain length. Oxygen-linked (O5O series) dimers show the highest SmCTB-SH transition temperatures, appearing at shorter chain lengths than their 6O analogues. The sulfur-linked dimers again give the lowest SmCTB-SH temperatures, consistent with reduced packing efficiency. Greater stabilisation of twist-bend phases with these more linear molecular structures is somewhat surprising, but longer chains enhance shape anisotropy and miocrophase separation, stabilising these smectic phases, while maintaining enough bend to induce the heliconal order required.
This detailed study of new dimers that exhibit both heliconical liquid crystal phases: twist-bend nematic and the rarer twist-bend smectic C shows how decisive chain length and terminal linking group are in determining phase properties. The central spacer also plays an important role, with diether spacers unexpectedly stabilising the SmCTB-SH phase, even though 6O spacers were more favourable for NTB. While both twist-bend phases require molecular bends, the layered smectic heliconal structure requires a careful balance of linearity, packing efficiency and microphase separation. This work work underscores the structure-property relationships at play in the creation of these twist-bend phases and what design rules can be employed to differentiate between NTB and SmCTB-SH phases.
Dr M. Tomsett – Technical Liason Officer
